BTW the Redmine mentioned PR (https://redmine.pfsense.org/issues/6626) is also available at github at https://github.com/pfsense/pfsense/commit/7c4b3d3c8d2d15b1e59d1d262cc295a848434355
So, the :: feature expands the $rule['interface']'s prefix to the host portion. Useless in my case.
Okay, lets make my target v6 a complete one: it works!
Assuming the Do not allow PD/Address release is being ignored and I get a new prefix, then all my rules are dead.
Correct me if Iam wrong, but pfSense misses a dropdown for that :: case, allowing me to select the target interface for auto-prefix-determination at https://github.com/pfsense/pfsense/blob/9fd4cb962ad28b0e03c8c755a80b20ad7c867d9e/src/etc/inc/filter.inc#L3247